1. Field of the Invention
The present invention concerns a method for recovering petroleum, especially viscous or heavy petroleum from subterranean formations by injecting hot fluids into the formation including low quality steam which is a mixture of liquid and gaseous phase components. More specifically, this invention concerns a method for injecting steam or mixture of steam and other additives into the formation in such a manner that the tendency for steam vapors to channel through the upper portion of the formation, bypassing substantial portions of the petroleum in the lower portion of the formation, is minimized.
2. Description of the Prior Art
Steam injection or steam flooding has gained substantial recognition in the art as a preferred method for recovering viscous or heavy oil from subterranean formations. For the purpose of the present application, it is meant by use of the term "heavy oil", petroleum which has an API gravity less than about 12.degree. API. Steam injection is generally applied to subterranean formations which have a low potential for production, e.g. less than about 10 percent of their initial petroleum by primary means, involving penetrating the formation with a well and pumping the petroleum contained therein to the surface of the earth without applying any treatment to formation petroleum to reduce its viscosity.
Steam may be used for oil recovery purposes in at least two general methods. In the first, steam is injected into one or more wells for a period of time, after which steam injection is terminated and petroleum is allowed to flow to the surface of the earth through the same well or wells as were used for injecting the steam in the formation. This cyclical procedure, sometimes referred to as "push-pull" steam stimulation, is an efficient method for simulating production from a well, but it is not satisfactory for exploiting a large aerial extent of a formation because the effect of steam injection diminishes in a push-pull sequence with distance from the point of injection due to heat losses and an ever increasing volume of depleted formation which must be saturated with steam before any new portion of the formation is contacted.
The second basic approach to steam injection is a well-to-well throughput process in which at least two wells are drilled into the formation and steam is injected into one well to pass through the permeable formation, displacing petroleum toward a remotely located well. This process has the advantage of being a continuous process in which petroleum production is not interrupted periodically as it is in the cyclical push-pull process. Moreover, the heating effect of steam is combined with the displacement phenomena similar to that employed in water flooding, which causes the creation of an oil bank between the wells which moves toward the production well and effectively displaces a substantial portion of the petroleum from the zone through which the steam moves in the formation.
When a well-to-well throughput steam injection process is applied to a thick reservoir, i.e. a subterranean petroleum saturated formation having vertical thickness of 50 feet or more, the vertical conformance of the steam process is relatively low. By vertical conformance, it is meant the portion of the vertical thickness of a formation through which the injected displacement fluid passes. Because steam is generally injected in a two phase form, at least a substantial portion being in the vapor phase, there is a strong tendency for the vapor phase component of steam to migrate to the upper portion of the petroleum reservoir. Horizontal vapor movement thereafter is confined to the upper portion of the formation, with the result that only a small percentage of the total vertical thickness of the formation is contacted by the vapor phase steam. Since the heat content of the vapor phase portion of steam is substantially higher than the liquid phase content it frequently occurs that only a small portion of thermal energy present in the injected fluid is used for decreasing the viscosity of petroleum and for recovering petroleum from the formation. Thus, it can occur in a throughput steam injection process that live, vapor phase steam exits from the production well, and yet a substantial portion of the formation between the wells, specifically the lower portions of the formation, has not been contacted by steam vapor.
In view of the foregoing discussion, it can be appreciated that there is a substantial, unfulfilled need for a method of conducting a well-to-well throughput steam injection oil recovery method in a manner which results in improved vertical conformance.